1. Field of the Invention
The invention pertains to a surface acoustic wave filter device. The invention particularly pertains to a bandpass surface acoustic wave filter having a relatively low insertion loss and high out-of-band rejection.
2. Description of the Prior Art
Surface acoustic wave devices have been used previously for bandpass filters. Often it was desired to provide a bandpass device having a low insertion loss for numerous applications such as in the RF stages of a radio frequency receiver. Attempts to achieve this goal included surface acoustic wave resonators and the use of unidirectional transducers. These are both low loss but the SAW resonator is necessarily a very small bandwidth and is difficult to design for good sidelobe suppression along with good passband shape. Unidirectional transducers suffer from a number of disadvantages depending on the technique used. For example, the earliest approach using two transducers driven in quadrature had very narrow bandwith and passband shaping could not be done. More recently, three phase transducers have been used but very complex fabrication involving multiple layers is required. That is, electrodes must be overlapped which will cause difficulty with fabrication yield and reliability.
In previous ordinary transversal surface wave filters, a 3 db loss was occasioned at both input and output transducers due to loss of half of the energy coupled into or out of the device by the transducer. With most devices, a surface wave is launched in each of two directions from the input transducer. Surface waves in only one of these directions ever reach the output transducer, the surface waves in the other direction being absorbed or dissipated at one edge of the device. At the output transducer, only approximately half of the energy was coupled into the output transducer. Thus, such devices had a minimum attainable insertion loss of at least 6 db not including the dissipation losses in the material itself.